In a recent interface device, forced or contacted feedback called haptic feedback is provided to a user. A haptic version of, for example, a joystick, a mouse, a game pad, a steering wheel, or other devices can output force to a user on the basis of an interaction or an event occurred in a graphic environment of a game or other application programs.
Meanwhile, a touch pad which is formed into a small square shape and provided at a mobile terminal can sense a position of a pointing object with various sensing techniques, such as a capacitive sensor and a pressure sensor, which sense a pressure applied thereon.
The haptic feedback is also provided at the touch pad of the mobile terminal.
FIG. 15 shows the structure of a conventional mobile terminal with a haptic feedback function disclosed in Korean Patent Laid-Open Publication No. 10-2001-0108361. As shown in FIG. 15, a touch pad 16 is elastically installed through a spring 62 at a housing (not shown) of a mobile terminal. A piezoelectric actuator 64 is disposed at a lower side of the touch pad 16. The piezoelectric actuator 64 is comprised of a vibration member (not shown) and a piezoelectric device (not shown) which is coupled to the vibration member. One portion 66 of the piezoelectric actuator 64 is coupled to the touch pad 16, and the other portion 68 thereof is coupled with a weight 70. And when the one portion 66 is moved with respect to the other portion 68, the weight 70 is moved together with the other portion 68. The weight 70 is moved approximately along a z-axis. Since the weight 70 is coupled to the housing, it can be moved freely.
The movement of the weight 70 along the z-axis causes inertial force transmitted to the touch pad 16 through the piezoelectric actuator 64, and the touch pad 16 is moved along the z-axis due to the elastic coupling. The movement of the touch pad 16 is sensed as a haptic sensation by a user contacted with the touch pad 16.
However, in a conventional haptic actuator 64, since the haptic feedback is embodied by only the inertial force of the weight 70, there is a problem in that the sensitivity of haptic feedback is deteriorated due to its weak up/down moving force.
FIG. 16 is a graph showing waveforms of a voltage and a current applied to a conventional piezoelectric device. As shown in FIG. 16, a square wave voltage is applied to a conventional piezoelectric device. In the square wave voltage, a momentary voltage is sharply increased. In FIG. 2, an input maximum voltage is 75V, and an output maximum current is 150 mA. Herein, a power consumption is expressed as P=V×I, and thus a momentary power consumption is 75V×0.15 A=about 11.25 W.
As described above, in the prior art, since the square wave voltage which is sharply increased is applied to the piezoelectric device, the vibration member and a panel part are suddenly vibrated and noise is generated. Further, since a slope of the input voltage is large, there is another problem that the momentary power consumption is also large.